Many golf balls are commercially selling, but they are typically classified into two-piece solid golf ball and thread wound golf balls. The two-piece solid golf ball consists of a solid core of molded rubber material and a cover of thermoplastic resin (e.g. ionomer resin) covering on the solid core. The thread wound golf ball consists of a solid or liquid center, a thread wound layer formed on the center and a cover of ionomer resin or balata etc. having a thickness of 1 to 2 mm covering on the thread wound layer. The two-piece solid golf ball, when compared with the thread wound golf ball, has better durability and better flight performance because of a larger initial velocity when hitting and a longer flight distance. The two-piece solid golf ball is generally approved or employed by many golfers, especially amateur golfers. On the other hand, the two-piece solid golf ball has poor shot feel at the time of hitting and poor controllability for approach shots because of a smaller amount of spin. The thread wound golf ball has better shot feel and controllability for approach shots compared to the two-piece solid golf ball, but less flight distance and less durability.
In order to provide a two-piece solid golf ball having a shot feel as good as the thread wound golf ball, a soft type two-piece solid golf ball using a softer core has been proposed. However, a use of the soft core adversely affects the rebound characteristics, thus resulting in a reduction in the of flight distance and a deterioration in the durability.
It has been proposed to place an intermediate layer between the core and the cover of the two-piece solid golf ball to maintain the balance between flight performance and shot feel at the time of hitting. For example, Japanese Patent Kokai Publication No. 194760/1984 suggests a three-piece solid golf ball in which the core is made harder from its center toward the surface and the specific gravity of the intermediate layer is higher than the core. The resulting golf ball has a soft shot feel, but the durability is not sufficient to endure repeated hitting with a golf club, because the hardness of the center point of the core is lower than that of the surface. Japanese Patent Kokai Publication No. 80377/1989 suggests a three-piece solid golf ball having a Shore D hardness difference between the core and the intermediate layer of not less than 20. However, since the difference in the hardness is too large, the difference in the deformation amount between the core and the intermediate layer is so large that the durability is extremely degraded when hitting the golf ball with a golf club. Accordingly, the durability is not sufficient to endure repeated hitting with a golf club.